Radiation curable compositions containing acrylic acid ester groups can be cured by exposure to ultraviolet light (UV). For a fast curing composition, a photoinitiator is necessary, which forms radicals under irradiation with photons and initiates free-radical polymerization of the acrylate groups, which then leads to a hardening (curing) of the product. Aldehyde-ketone photoinitiators are described in US 2009/0048363.
However, radiation curable compositions containing photoinitiators can be a challenge for several applications in packaging, especially in food packaging (“Radiation curing in packaging”, Radtech Report March/April 2006). Residual photoinitiators, as well as their cleavage products, remain in the coating or ink and may cause problems, such as for example migration, contamination, odor or off taste. For example, residues of photoinitiators and cleavage products can migrate through the substrate and affect adjacent products such as food stuff or may contaminate food by off set contact migration. Therefore, there is a continuous search for photoinitiator systems which have low potential for migration and odor.
As described in the literature (L. L. Katan in “Migration of additive food contact”, Black Academical & Professional, first edition, London 1996, page 97, table 5.3), the ability of a material to migrate is governed by its molecular weight, rather than by its chemical structure. Therefore, for a low migration UV-curable ink or coating, photoactive components having a molecular weight of >500-1,000 Daltons are preferred.
Oligomeric and polymeric tertiary aromatic amines are used in the art to enhance the UV-curing response of radiation curable compositions (“Industrial Photoinitiators”, CRC press London 2010, page 106). Together with ketones, they can form type-II photoinitiators. Tertiary amines are especially useful for the surface cure of UV-inks and UV-coatings as they are able to transform non-reactive oxo-radicals, which are deactivated by reaction with oxygen, into more reactive radicals and reduce the inhibiting effect of oxygen on the polymerization on the surface of inks and coatings, (“Photoinitiators for free radical and cationic & anionic photo-polymerization”, Wiley & Sons 1998, page 84).
Moreover, unlike type-I photoinitiators, type-II photoinitiators usually do not form small molecules (splitting products) and are therefore especially useful in applications requiring low migration of chemical species.
Among the most reactive amines proposed for low migration applications are oligomeric aminobenzoates. Examples of oligomeric aminobenzoates for low migration applications are, for example, Genopol AB-1 and AB-2 (products of Rahn Group, Switzerland) and Omnipol ASA (product of IGM Resins, Netherlands). However, aminobenzoates still need a partner to form an effective initiator system, such as an aromatic ketone or thioxanthone.
In contrast, aromatic aminoketones, besides the tertiary amino group, already contain an aromatic ketone, which can be excited by UV-light and additionally contribute to UV-cure. An example of an oligomeric aminoketone for low migration applications is Omnipol SZ (IGM Resins).
Moreover, due to recent developments of UV-bulbs emitting predominantly in the UV-A area, and which do not produce ozone and exhibit a low energy consumption, there is a constant search for photoactive materials especially useful for UV-A curing to make the UV-technology safer in relation to both migration and process safety.